Gear tooth sensors are mainly used in automatic control systems to measure the speed and direction of rotation of a gear. Presently, the most commonly used gear tooth sensors utilize optical or magnetic sensing. In rotating mechanical systems harsh conditions such as vibration, shock, oil, and etc. are present which are not well tolerated by optical sensing, and do not affect magnetic sensors, so magnetic sensors have advantages over optical sensors in these systems. There are many different types of magnetic sensors used in prior art as the magnetic sensor element of a magnetic gear tooth sensor, including Hall (Hall) effect sensors, anisotropic magnetoresistance (AMR) sensors, and giant magnetoresistance (GMR) sensors.
Hall Effect sensors have very low sensitivity, typically requiring the use of a flux concentrator to increase sensitivity of the sensor which also increases size and weight. In addition, the Hall sensor element is a sensor has high power consumption and concentrators can exhibit poor linearity. AMR elements have higher sensitivity than Hall Effect elements, but suffer from much narrower linear working range. AMR element's also require a ‘set/reset’ coil used to reduce hysteresis, which not only leads to a more complex manufacturing process, hut also increases AMR sensor size and power consumption. GMR sensors have higher sensitivity than AMR elements, but also suffer from narrow linear working range. Further, the response curve of a multilayer element is unipolar, and multilayer GMR elements cannot measure the polarity of the magnetic field.
In recent years, a new type of magnetoresistive sensor, known as a magnetic tunnel junction (MTJ) have begun to find acceptance for use as magnetic sensors in industrial applications. The working principle of an element is based on the use of the tunneling magnetoresistance effect (TMR) in magnetic multilayer films. MTJ elements show much higher magnetoresistance than AMR or GMR elements. Compared with the Hall Effect, an MTJ sensor has better temperature stability, higher sensitivity, lower power consumption and better linearity, and requires no extra flux concentrator structure to improve sensitivity. Compared with AMR sensors, MTJ sensors have better temperature stability, higher sensitivity, wider linear operating region, and they do not require the extra ‘set/reset’ coil structure. Compared with GMR sensors. MTJ sensors have improved temperature stability, higher sensitivity, lower power consumption, and a wider linear operating range.
Magnetic gear tooth sensors typically use a printed circuit board (PCB) based structure to support the components. PCB based gear sensor is usually comprised of a magnetic sensor chip, some circuitry, and a permanent magnet. The permanent magnet produces an applied magnetic field Happly, which produces a change in the presence of a gar tooth that the magnetic sensor chips detect and then output a proportional voltage signal; the peripheral circuit is used for signal processing and conversion of the sensor output into an appropriate signal. The applied field generated by the permanent magnets produces a weak Happly at the physical location of the magnetic sensor chip along the sensing direction, which limits the amount of field it can be designed to deliver. As a result, for the PCB type gear tooth sensor, improved suppression of external interference and increased Happly are technical challenges that remain to be solved.
Although MTJ elements have very high sensitivity, they have the following issues:
(1) External magnetic field generated by the permanent magnets along the sensitive direction of the MTJ element Happly is too large, causing the MTJ element to exhibit nonlinear performance, or worse still saturating the MTJ element;
(2) The magnetic field at the physical location of the sensor chips produced by the permanent magnets Happly and the external magnetic field can change, making the MTJ elements vulnerable to outside magnetic field interference in addition to drift in Happly produced by the permanent magnet;
(3) Inability to determine the position of a gear tooth or the existence of missing gear teeth;
(4) No method to determine the direction of movement of the gear;
(5) Low cost mass production has not yet been achieved.
Therefore, a need to improve magnetic gear tooth sensor technology to accurately sense the motion and health of gears.